why is 'child of' modifier scaling my bone?
A child of constraint is the same as parenting an object, when you move the parent the child also moves, scale the parent and the child also scales.

By using the constraint instead of direct parenting you can control some of this behaviour. By turning off the scale options you can stop the lightsaber scaling with your armature.



This does highlight one point though, you have an armature that is scaled, you most likely scaled the armature while in object mode to get it to fit your model. Having an armature that is scaled can cause odd behaviour like this at times so you may want to apply the scale to your armature by pressing CtrlA and choosing scale while in object mode.